Prior related closures of this general type may be seen, for example, in U.S. Pat. Nos. 3,893,583, 4,037,746, 4,066,182, 4,202,455, and 4,589,561. There are several problems with these prior art closures, however.
Maximum dimensional control of blow molded, plastic containers is achieved at the outer surface of the container, which is formed in contact with the mold. The inner edges of the mouths of blow molded containers are trimmed, and because the trimming operation is not precise, the edges are not necessarily flat and free of burrs or other formations which would effect their dimensions. Consequently, any seal, such as a plug seal, that attempts to seal a container at these inner edges is subject to the non-uniformity in dimension caused by the trimming operation. A much more dependable seal may be achieved at, for example, the outer rim of the mouth of a container where the molded dimensions are more uniform.
Some prior art devices attempt to seal containers along a trimmed edge rather than a molded outer surface. This makes the device prone to leakage. Some that attempt to seal at the outer surface of containers do so inadequately, and are known to leak during shipping and storage of the containers. This offends consumers and makes the products undesirable and sometimes unsaleable. Some achieve adequate sealability at the expense of aesthetic, cost, and ease-of-use considerations. Still others are configured such that they are difficult to remove from molds during their manufacture.
There has therefore been a desire to have a tamper-evident closure for containers, including improved sealability, which eliminates leakage during shipping, storage, and handling, while remaining aesthetically pleasing to consumers, easy to use, and inexpensive to produce. There is also a desire to provide such a closure, including a frangible pull-tab for easily releasing and removing the closure from containers, and which is relatively easy to remove from molds during manufacture.